The Nature of Interface Interactions Leading to High Ionic Conductivity in LiBH4/SiO2 Nanocomposites

Complex metal hydride/oxide nanocomposites are a promising class of solid-state electrolytes. They exhibit high ionic conductivities due to an interaction of the metal hydride with the surface of the oxide. The exact nature of this interaction and composition of the hydride/oxide interface is not yet known. Using 1H, 7Li, 11B, and 29Si NMR spectroscopy and lithium borohydride confined in nanoporous silica as a model system, we now elucidate the chemistry and dynamics occurring at the interface between the scaffold and the complex metal hydride. We observed that the structure of the oxide scaffold has a significant effect on the ionic conductivity. A previously unknown silicon site was observed in the nanocomposites and correlated to the LiBH4 at the interface with silica. We provide a model for the origin of this silicon site which reveals that siloxane bonds are broken and highly dynamic silicon–hydride–borohydride and silicon–oxide–lithium bonds are formed at the interface between LiBH4 and silica. Additionally, we discovered a strong correlation between the thickness of the silica pore walls and the fraction of the LiBH4 that displays fast dynamics. Our findings provide insights on the role of the local scaffold structure and the chemistry of the interaction at the interface between complex metal hydrides and oxide hosts. These findings are relevant for other complex hydride/metal oxide systems where interface effects leads to a high ionic conductivity.

1 Synthesis protocols 1.1 Mesoporous silica scaolds  The primary mesopore diameter of Santa Barbara Amorphous-15 S1  can be varied by tuning the hydrothermal synthesis temperature. S2 A mixture of poly(ethylene glycol)-block-poly(propylene glycol)-block-poly(ethylene glycol) (P123), hydrochloric acid (37 %) and water was dissolved at 35 • C. Tetraethyl orthosilicate (TEOS) was added dropwise and the mixture was stirred at 40 • C for 24 h. The molar ratios were 0.015:5.2:147:1 for P123:HCl:H 2 O:TEOS respectively. Hydrothermal treatment was done by heating the mixture to 40, 70, 85, 100 or 120 • C in a closed bottle or autoclave for 44 h, after which the solid precipitate was washed with water. The product was dried, ground, and subsequently calcined in air by increasing the temperature to 550 • C with a ramp of 1.2 • C/min and kept at this temperature for 12 h. The silicas are referred to as SBA-15 followed by their hydrothermal treatment temperature.  The pore diameter of Mobil Composition of Matter 41  was tuned by the choice of surfactant. Synthesis was performed as described by Cheng et al. S4 Thick-walled MCM-41 was synthesized by using elevated hydrothermal treatment temperatures (170-180 • C). S5 A mixture of water, n-alkyl-trimethylammonium bromide (C n TAB, n=10,14,16) and tetramethylammonium hydroxide (TMAOH, 25 %) was dissolved at 30 • C. Aerosil was added and the mixture was stirred for 2 h, then left standing at 30 • C for 20 h. The molar ratios in the mixture were 39:0.26:0.18:1 for H 2 O:C n TAB:TMAOH:SiO 2 respectively. Hydrothermal treatment was done by heating the mixture to 150, 170 or 180 • C in an autoclave for 44 h. The MCM-41 was subsequently washed and calcined as described for SBA-15. The silicas are referred to as MCM-41 followed by the C n (TAB) chain length and the hydrothermal treatment temperature; the sux`-2' is used to dierentiate scaolds prepared using the same procedure.

LiBH 4 /SiO 2 nanocomposites
All silica scaolds were dried at 300 • C in a glass reactor under a constant ow of inert gas, using a ramp of 5 • C/min, and kept at that temperature for 6 h. Subsequently, the dried silica was stored in gloveboxes with H 2 O and O 2 levels typically ≤ 1 ppm. The LiBH 4 /SiO 2 nanocomposites were prepared via melt-inltration of the dried silica scaolds. S6 LiBH 4 (95 %, Sigma-Aldrich) and dried silica were hand-mixed in the glovebox. The LiBH 4 loadings (fraction of pores that would be lled with LiBH 4 if all LiBH 4 inltrates) can be found in tables S2 and S3 for the samples used for impedance spectroscopy and NMR experiments respectively. These mixtures were transferred to borosilicate glass vials and placed in a stainless-steel autoclave. Approximately 50 bar H 2 gas was applied and the autoclave was heated to 300 • C using a ramp of 3 • C/min, and kept at this temperature for 30 min. After cooling down, the H 2 pressure was released and the samples were transferred into the glovebox without exposure to air. Nanocomposites used for studies by NMR are named by their scaold suxed by i or ii to distinguish nanocomposites of dierent batches that use the same silica scaold.
S-2 Table S1: Structural properties of the silica scaolds.  S1 . For SBA-15, the DFT model for cylindrical pores is less applicable, resulting in negative thickness values.
S-3 ). e Apparent activation energy of the Li-ion hopping process, determined from a linear t of the Arrhenius plot between ambient temperature and 100 • C (gure S5). f A checkmark indicates whether the nanocomposite was included in Figure 1. Nanocomposites with excessive LiBH 4 loadings, thick walls (compression dierences, see gure S17) or dierent pressures were excluded from this gure. g The pressure for making the pellet consisting of lithium foil|nanocomposite|lithium foil, which was subsequently used in impedance spectroscopy. h Fumed silica lacks a well-dened pore structure. Hence, the fraction of LiBH 4 that does no longer undergo the bulk phase transition around 110 • C is shown instead. that does no longer undergo the bulk phase transition around 110 • C is shown instead. Figure S1: N 2 -physisorption isotherms of the silicas. Adsorption isotherms are shown in red, desorption isotherms in blue. An arbitrary oset was applied for clarity.
S-6 Figure S2: Pore size distributions of the silicas according to the classical BJH model S9 applied to the adsorption isotherm (gure S1).
S-7 Figure S3: Normalized low angle powder X-ray diraction patterns of the silica scaolds. The major peaks of MCM-41 and SBA-15 correspond, from left to right, to the (100), (110) and (200) reections, respectively. Higher reections are visible for some silica scaolds. Aerosil has no long-range ordering and is hence featureless. An arbitrary oset was applied for clarity. The sloping baseline on the left is an artifact due to direct beam exposure.
S-8 Figure S4: Transmission electron micrograms of (a) MCM-41-C 16 -150 and (b) MCM-41-C 16 -180, showing the ordered porous structure of both silica scaolds. Mesoporous silica scaolds are very prone to electron beam damage S11 , hence dierent maximum zoom levels were used. Some beam damage is visible in b).   For assignment of the peaks, the reader is referred to gure 3 and the main text; the ratios of the peaks are listed in table 2. The experiments were performed on a eld of 7.05 T at 3 and 6.5 kHz MAS respectively. The spectrum of the silica (a) is the sum of spectra with a xed delay of 50000 and 86400 s between subsequent scans. The 29 Si spin-relaxation time, which is longest for the Q 4 peak, is T Q 4 1 ≈ 2 · 10 4 ± 1 · 10 4 s in this silica scaold. The spectrum of the nanocomposite (b) was recorded with a xed delay of 3600 s between scans; at longer delays between scans, (only) the Q 4 peak will gain more intensity (4 to 8 percent points), presumably due to silica that is at a large distance from LiBH 4 . Figure S11: { 1 H} 29 Si CP-MAS NMR spectra of silica of the type SBA-15, before and after being treated under melt-inltration conditions (without LiBH 4 ). The spectra display excellent agreement, indicating no changes have occurred as a result of this treatment. The spectra were recorded on a eld of 9.4 T at a spinning speed of 3.25 kHz. The weak spinning sidebands are indicated by asterisks.
S-14 Figure S12: 1 H NMR spectrum of silica scaold MCM-41-C 16 -150-2 (without LiBH 4 ), measured using a Hahn echo at a eld of 9.4 T under 3.25 kHz MAS. Spinning sidebands are indicated by asterisks. The peaks between 1.6 and 2.2 ppm are ascribed to silanol groups. S14 S-15 Table S4: Dipolar second moments and corresponding (r −3 -weighted) average internuclear distances obtained from ts of { 7 Li} 29 Si and { 11 B} 29 Si REDOR curves of the nanocomposite MCM-41-C 16 -150-2 i (gure S13). The REDOR curves were tted to the analytical formula derived by Hirschinger to obtain the heteronuclear dipolar second moment and the average internuclear distance. S15S17 As the quadrupolar interactions of 11 B and 7 Li in LiBH 4 / SiO 2 nanocomposites are small, the REDOR experiment can be used quantitatively. S18 The margin in the least-squares t of the second moment is typically 20-30 %, resulting in an error of approximately 0.3 Å in the distance. This excludes the error in the REDOR curve, and assumes each lithium or boron atom couples with only one silicon atom of a certain species. The data of Q ′ 3 may include a contribution of isolated SiOH or SiH sites, but this contribution is expected to be small.

Nuclei
Second moment (kHz 2 ) Internuclear distance (   peaks. Spinning sidebands of LiBH 4 are indicated by asterisks. The small peaks between 20 and −35 ppm correspond mostly to oxidations of LiBH 4 as demonstrated in the supporting information of our previous study. S12 The highly dynamic and more bulk-like LiBH 4 fractions resonate at −40.5 and −41.3 ppm, respectively. S19 The chemical shift of the bulk-like fraction is equal to the chemical shift of bulk LiBH 4 . S20 Figure S17: Change in volume of the silica upon applying pressure. The volume change was measured in-situ in a 13 mm pellet die. The silicas shown are MCM-41-C 16 -150-2 (thin-walled MCM-41), MCM-41-C 16 -180-2 (thick-walled MCM-41) and SBA-15-120. For comparison: the volume change of Aerosil 380 (not shown) was about −8 mL/g at a pressure of 38 MPa, then stabilizing at −8.5 mL/g at higher pressures.

S-18
Semi-quantitative approach to determine whether the mii -peak is proportional to the interface between LiBH 4 and silica or to the total composition of the nanocomposite.
Although CP-MAS experiments are not inherently quantitative, it is possible to do a rough quantitative comparison of the mii-peaks in the nanocomposites MCM-41-C 16 -150-2 i (thin pore walls) and MCM-41-C 16 -180-2 i (thick pore walls), as the cross-polarization dynamics are comparable for both nanocomposites (gure S15).
After normalization to equal number of molecules per NMR rotor (using the 7 Li integrals and the sample composition) and the number of scans, the ratio between the mii-peaks of the nanocomposites with thin and thick silica pore walls was: We now consider two hypothetical cases: 1. The mii-peak is proportional to the surface area of the silica. In this case, the peak would be expected to scale with the surface area (BET , m 2 /mole) of the silica scaold, and the molar ratio between silica and LiBH 4 (f SiO2 = n SiO2 ÷(n LiBH4 + n SiO2 ) ∝ n SiO2 , the latter step due to normalization), i.e., doubling of the surface area per amount of silica or doubling the amount of silica (which would both double the total surface area) would result in a doubling of the mii peak. This assumes sucient LiBH 4 is present to wet the entire silica surface. This hypothetical case would yield a ratio of: 2. The mii-peak is proportional to the total amount of silica. In this case, the peak would roughly scale with the fraction of silica in the nanocomposite. This case would yield a hypothetical ratio of: Clearly, the hypothetical ratio for an interaction involving the total amount of silica is much closer to the experimentally determined ratio, than an interaction involving only the interface between lithium borohydride and silica. It should be noted that these are rough approximations: i.e. pore lling deciencies are not taken into account.